1. Field of the Invention
This invention relates to fusible or fail-safe resistors generally, and specifically to controlled thermal destruction of the substrate upon which a resistor is carried.
2. Description of the Related Art
Prior art fusible or fail-safe resistors utilize several general techniques to produce an electrical and/or mechanical disconnection of power when the resistor is electrically overloaded. These techniques include the physical addition of a separate and distinct fuse directly adjacent to the resistor, the combination of a fuse link or fuse element adjacent with or integral to the resistor, the use of a thermally sensitive substrate or device which when thermally stressed produces some expansion resultant in the disconnection of power to the resistor, or the use of a controlled resistance film thickness which when overloaded evaporates.
Prior art devices which utilize a separate and distinct fuse element are inherently costly since two devices need to be placed rather than one, resulting in a higher cost of materials, higher cost of production, lower end product reliability, and increased component space consumption.
Those devices which employ a fuse or fuse link integral to the resistor also face similar cost problems. Again, the fuse or fuse link requires extra materials and processing steps which are primarily exclusive to the fuse. The major advantage gained in this type of system is the reduced real estate required for the fuse link. Additional advantage may be gained in the sensitivity of the fuse to the thermal status of the resistor due to the close proximity of the fuse. A fuse of this type is illustrated in U.S. Pat. No. 4,494,104 to Holmes. Illustrated is a gold/solder fuse link spanning a gap between two resistor bodies, all on a common substrate. The fuse link is sensitive to the temperature of the common substrate, and breaks connection when this substrate reaches a sufficient temperature.
Another type of fusible resistor is a controlled film thickness resistor. This type of resistor may be either vapor deposited or screen printed or produced by any of a variety of other known techniques, but it is always produced so as to have a controlled cross-section through which the current must flow. This controlled cross-section of resistance material then vaporizes upon excessive heating. This type of resistor is difficult and expensive to manufacture and usually has poor reliability characteristics. In order to overcome the variables in typical discrete fuses, which operate on a similar principle, the discrete fuses are glass encapsulated. Such encapsulation would clearly increase the cost substantially for a typical resistor.
Additional complex devices are known in the art which include bimetallic strips or other types of thermally sensitive mechanical devices to control the application of electrical energy to the device, such as are illustrated in U.S. Pat. Nos. 3,763,454 to Zandonatti and 2,263,752 to Babler, but do to the complexity, these devices are also inherently expensive and raise reliability concerns.
Additional devices which employ thermally responsive substrates are disclosed in the prior art. Such devices include substrates which shatter upon excessive heating, such as disclosed by Lytle in U.S. Pat. No. 2,730,598. Another variation is disclosed by Harmon et al in U.S. Pat. No. 4,208,645 in which a substrate material is disclosed which expand along a single axis differently from the other two axes, sufficiently so that a circuit trace connecting the resistance material to the source of electrical energy may be separated.
Dennis et al in U.S. Pat. No. 3,978,443 disclose a resistor having long conductors which cross a path on a porcelain substrate which is identified as being the most likely region of substrate failure. The substrate then breaks along a .scribed mark positioned to correspond to this most likely region upon overheating of the resistance material and substrate. This method represents the most reproducible of the prior art methods for causing a resistor to fail at a controlled energy dissipation point, but still suffers from several drawbacks. First, the scribe must either be placed entirely across the device, or in the alternative embodiment, must pass entirely through a portion of the substrate, in each case in a zone predetermined to be the most likely for thermal device failure. If the scribe passes across the entire surface of the substrate, then the resistance material must be coated directly on top of the scribe, resulting in a much more difficult and less reliable resistor. If the scribe passes through the device, it requires custom molded substrates (higher cost) and does not result in particularly controllable results.